The present invention relates to nitrogenated evaporable getter devices with high fritting resistance and to a process for their production.
It is known to use evaporable getter devices to maintain a vacuum inside kinescopes of the type that are used as the CRT of a television set and as the monitor of a computer screen. Evaporable getter devices are also being developed for use in flat panel display screens.
The evaporable getter devices that are used in kinescopes comprise a thin layer of barium that is deposited onto an inner wall of the kinescopes. The barium layer is active in gas sorption.
Evaporable getter devices are generally formed from an open metal container that contains a barium and aluminum compound (BaAl.sub.4) in the form of a powder and nickel (Ni) in the form of a powder in a weight ratio of about 1:1. Once the kinescope has been evacuated and sealed, the barium is evaporated by induction-heating the evaporable getter device using a coil arranged outside the kinescope. This activation process is commonly known as "flash" activation. The following reaction takes place when the temperature of the powders reaches a value of about 800.degree. C.: EQU BaAl.sub.4 +4Ni.fwdarw.Ba+4NiAl (I)
This reaction is strongly exothermic. The temperature of powders reaches about 1200.degree. C., the barium that is released evaporates and is deposited by sublimation onto the walls of the kinescope.
It is known to obtain better barium films by having a small partial pressure of gas during the "flash" operation. Nitrogen gas is generally used to provide the partial pressure. These conditions encourage the evaporated barium to be homogeneously spread in all directions such that the resulting thin layer of metal has a more uniform thickness and a larger surface area.
Evaporable getter devices can contain small amounts of a nitrogenated compound in addition to BaAl.sub.4 and Ni. Getter devices of this type are indicated as "nitrogenated". These nitrogenated getter devices release a small amount of nitrogen during the "flash" operation. The thin layer of barium resorbs the nitrogen within a few seconds. Compounds that are commonly used in nitrogenated getter devices include iron nitride (Fe.sub.4 N), germanium nitride (Ge.sub.3 N.sub.4) or mixtures thereof.
Producing kinescopes, both conventional CRT types as well as for flat panel displays, involves welding two glass members in a "fritting" operation. This process involves forming a paste of glass that is molten or softened between the two members in presence of air. The glass paste has a melting point of about 450.degree. C.
The evaporable getter device can be introduced into a conventional CRT-type kinescope after the fritting through the neck intended for housing of the electron gun. However, the diameter of the neck limits size of the getter device such that accurately positioning the evaporable getter device in the kinescope becomes difficult. For flat panel display screens, it is almost impossible to introduce the evaporable getter device after the fritting operation. Thus, kinescope manufacturers need to be able to introduce the evaporable getter device before the fritting operation.
The fritting operation simultaneously, but indirectly, exposes the evaporable getter device to heat, to atmospheric gases and to vapors that are discharged by the low-melting glass paste. These conditions cause a superficial oxidization of the components of the evaporable getter device. The extent of this oxidization depends on the specific compound. This oxidation results in a strongly exothermic reaction during the "flash" operation that is difficult to control. This reaction may result in the powder packet being uplifted, in fragments thereof being ejected, or in the container being partially melted. Any of these undesirable effects can interfere with the proper functioning of the getter device and compromise the operation of the kinescope. Furthermore, the partial oxidization of the getter during the fritting operation can cause the nitrogenated compound to undergo a partial loss of nitrogen.
Evaporable getter devices that resist partial oxidization during the fritting operation without, or that do not exhibit, the aforementioned disadvantages, are called "frittable".
The fritting operation generally lasts between one and two hours. Evaporable getter devices that are frittable under these conditions are known.
For example, U.S. Pat. No. 4,077,899 discloses non-nitrogenated evaporable getter devices that are made frittable by using nickel powders that have a particle size ranging from 30 to 65 .mu.m, rather than the smaller particle size, generally smaller than 20 .mu.m, generally used for non-frittable evaporable getter devices.
U.S. Pat. No. 4,127,361, which is assigned to the assignee of the present application, discloses getter devices that are made frittable using a protecting layer of organosilanes. This coating reduces the extent to which the nickel is oxidized, and may also limit the oxidation of Fe.sub.4 N. Accordingly, the aforementioned problems are eliminated during barium evaporation. However, it has been found that the coating process required by this method is too slow to be compatible with the industrial production.
French Patent No. 2,351,495 discloses coating the whole getter device with a thin layer of silicon oxide obtained by hydrolysis of an organic silicate. However, the resulting layer of silicon oxide is porous and, though reducing it, does not avoid the nickel oxidation. This limitation has disadvantages even for fritting times of less than two hours.
U.S. Pat. No. 4,342,662 discloses getter devices protected by a thin vitreous layer of a boron compound selected among boron oxide and boric acids, possibly further containing silicon oxide in an amount not greater than 7% by weight. These devices withstand the fritting at 450.degree. C. for a time up to two hours.
Finally, Japanese laid-open Patent No. Hei-2-6185 discloses the protection at least of the nickel by means of a protecting layer made of boron oxide only.
However, in some cases, irregularities in the production cycle of kinescopes may result in fritting times lasting up to about five hours, either as continuous steps or as sum of the times from consecutive fritting steps. Known nitrogenated evaporable getter devices are not capable of withstanding fritting for so long.